Drug therapy has thus far attained little success in managing cancers, primarily due to a lack clinical efficacy and presence of drug-induced toxicity. In the previous project, we instituted principles to overcome such obstacles in achieving an ideal drug therapy of maximum therapeutic efficacy with minimal drug-induced toxicity. This drug delivery system (DDS) is comprised of two components: [1] a targeting part composed of heparin linked to a mAb, and [2] a drug part, comprised of a drug coupled with a cell penetrating peptide (CPP). These two components can self-assemble into a tight complex via charge interaction between heparin and CPP. Binding CPP with heparin would mask its cell-penetrating function due to inhibition of its adsorption to the cell surfaces. The drug complex provides a prodrug behavior and prevents CPP-mediated uptake by normal tissues, alleviating drug-induced side effects. Following tumor targeting via the mAb, protamine will be administered to unmask heparin inhibition and restore the activity of the released CPP-Drug, inducing apoptosis to only tumor cells. Exceptional progress was made during the past grant period, with publication of 36 papers and graduation of 12 PhD/Postdoctoral trainees. All aims were accomplished, and utility/applicability of this DDS was confirmed. Despite promise, two crucial factors - efficacy of antibody targeting and extent of prodrug protection - were found in need of improvement in order to realize a clinically enabled drug therapy. Recent advancement in colorectal cancer research seems to offer a solution to these two issues. First, carcinoembryonic antigen (CEA), a surface antigen known to associate with tumor growth/metastasis, was shown to be overly expressed in colonic cancers but limited in normal tissues. Secondly, we discovered that >85% of the injected T84.66, a mouse mAb against human CEA, accumulated at the colorectal tumor target via an antigen-mediated elimination pathway. Thirdly, the protease legumain was recently reported to express profoundly on the surface of colorectal tumors but is almost undetectable in normal tissues. Using this information, we further engineered a highly improved DDS to cross the afore-cited barriers. Briefly, a potent toxin (gelonin) or siRNA drug will be covalently linked with LMWP, a proven but non-toxic CPP, via S-S bond. This LMWP-Drug will be further linked to T84.66 through a peptide linker containing the legumain-degradable specific AANL sequence. The drug complex would then exhibit a prodrug behavior during tumor targeting, since LMWP is embedded between the antibody and drug molecule, thus unable to transverse the drug through tumor cells in exerting cytotoxic effects. Following tumor accumulation via T84.66 targeting, release of LMWP-Drug from T84.66 will take place on tumor surface via cleavage of the AANL linker by surface-expressed legumain. LMWP-Drug will then internalize into tumor cells via LMWP-mediated cell entry action, initiating tumor apoptosis. The specific aims are to validate the pre-clinical capability of this system in managing specifically colorectal cancers, through extensive in vitro and in vivo studies using a well-established LS174T xenograft mouse model; thus paving the road for future clinical translation.